Hi there, welcome! And thank you for adopting!
So with youngsters that are together as babies, if they are spayed/neutered young enough (before hormones set in fully), they are often OK to stay together. Shelters usually spay/neuter really young, so the chances they can stay together are better and they don’t experience as much of a change after they are castrated. That said, sometimes a change in environment can upset things a bit.
I think the best thing to do would be to keep them together, but have a back-up enclosure ready to go in case any fighting occurs (perhaps the shelter could even loan you an x-pen and litter box). If they are happy together now at the shelter, they will find a lot of comfort in each other going to a new home and will be happier together. There are usually warning signs that the bond has been upset. Like you might notice some chasing, nipping, fur pulling. In that case I would separate but keep them side by side, and then you can work on re-bonding in a few weeks once hormones have calmed more.
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The answers provided in this discussion are for general guideline purposes only. The information is not intended to diagnose or treat your pet. Seek the advice of your veterinarian or a qualified behaviorist.